Show build time

Show build time:

$ ./ninja --buildtime
ninja: no work to do.
build time: 00:00:00.000 (0 ms)

$ ./ninja --buildtime
[23/23] LINK ninja
build time: 00:00:00.113 (113 ms)
Is it available on all platforms? At the moment `win32port.h` was used (maybe unnecessary)?